Photolithography is a technique for exposing and imaging a mask pattern onto a substrate, which plays an important role in the fabrication of semiconductor devices. During the fabrication of a semiconductor device, it is necessary to provide an imaging system in the photolithography tool with an illumination field of view (FOV) that has a desired size and a light intensity uniformity. At present, such desired light intensity uniformity is realized by an optical system with a light homogenizer such as an optical integrator rod or a microlens adopted in the design of the imaging system. This optical system allows the formation of a uniform illumination FOV on an objective surface.
During the design of an illumination system, the requirements for a desired light intensity distribution in the illumination FOV must be satisfied. However, errors may be introduced in the subsequent processes of lens processing, coating and mechanical installation due to some uncontrollable factors. As a result, the actual final light intensity distribution may more or less deviate from what it is purported to be. In this case, it will be necessary to take some effective measures to modulate the actual light intensity distribution in order to compensate for the deviation and increase accuracy of the illumination FOV.
In addition, after the illumination system is completed, it is also needed to modulate the illumination FOV in order to make it suitable for use in more applications. The modulation of the illumination FOV is essentially that of its light intensity distribution, for example, the modulation of the trend, uniformity or the like. Moreover, with the continuous development of the photolithography technology, there is an increasing demand for post-manufacturing adjustments in light intensity distribution. Therefore, there is a need for a reliable, accurate method for adjusting the light intensity distribution of an illumination FOV such that the illumination FOV of the illumination system may have a wider range of applications.
In the prior art, there are two available solutions for modulating a light intensity distribution. FIG. 1 shows one of the solutions, which enables unidirectional light transmission modulation by placing an assembly of individual shading plates 10 on the optical path of an illumination system. The shading plates 10 are each provided with opaque spots 11. The opaque spots 11 on the shading plates 10 can be combined and superimposed in different manners to adjust the range and transmittance in light intensity modulation. This technique, however, has a number of deficiencies such as only allowing the modulation of transmittance, a narrow adjustable FOV size, a low adjustment accuracy and a high structural complexity. The other one of the conventional solutions employs a filter constructed by coating (e.g., plating) films with different transmittances on different regions of a transparent substrate. In this way, the regions coated with the films also have different transmittances and enable light intensity modulation. This technique also suffers from several disadvantages such as a low accuracy, different film-coating designs required for intensity modulation of light having different wavelengths, a high cost and a high manufacturing complexity. Further, both of the above two conventional techniques are applicable to a limited range of wavelengths.
In summary, the conventional techniques are associated with drawbacks such as a low modulation accuracy, a small FOV size of modulation, a narrow light intensity modulation range, a small applicable wavelength range and a high manufacturing complexity.